PTC Heater installation
The last system I had to engineer for the car was a heating system. I had originally planned to use a HotStart water heater and use the original piping and controls in the car. The HotStart is a 5kW unit and probably would provide great heating capacity. Unfortunately there is just no room for the unit in the engine compartment. As part of completing the conversion I started placing all the components in the engine compartment to figure out where they would go and how the components would be mounted. I found there is no room for the HotStart because I need two cooling loops, one for the DMOC and one for the Siemens. A cooling loop consists of a water reservoir and a pump that goes to the heat exchanger. The HotStart would also need a water reservoir and a pump to circulate the water through the heater core. My only other option for heating is to use electrical resistance heating with a PTC heater. PTC stands for positive temperature coefficient, which for a heater means that the resistance increases as the heat increases so the PTC heater is self limiting. From doing some research I found that I probably would need at least 3kW of heating. PTC heaters are great because the heat up fast, but have the downside of not have a great heat capacity. Searching on EBay I found there are all shapes and sizes of PTC heaters. The ones I chose are a 1kW unit rated for 120VAC. Fortunately three of the units side by side is nearly exactly the same size as the old heater core. By using three wired in series I can get 3kW of heating and use the pack voltage to drive the current. Testing the units I found they started drawing 9 Amps at 120VAC and as they heated up the current dropped to 5 Amps. The units have a thermostat mounted on the side with a 160C cutoff temperature. I found the using a thermocouple attached to the heating element the thermostats open around 170C. Taking apart the old heater core was a bit of a challenge. The whole unit, heater core, fans and air distribution box come out of the car as a single unit that is held together is clip rings. Taking apart the unit I found all of the foam insulation and gasket material completely degraded and falling apart. The inside of the air box was also very dirty. Back in 1983 they did not use any air filters on the incoming air like they do now. The X1 BMW I have actually has a HEPA air filter for the cabin air. I will probably look at adding some type of air filtration when I reassembly the unit. Because it was so dirty I took all the electrical wiring and connections out and washed the whole unit. During the disassembly I discovered how the fan speed was set. They used a set of big wire wound series resistors to control the current to the fan. Two resistors would give four speeds for the fan and that is how many selections there are on the dashboard switch. I will replace those resistors with a MOSFET circuit to control the fan speed. I reassembled the air box with the PTC heater inside and mounted a few thermocouples inside to measure the heat. The PTCs were wired in parallel because I do not have the battery pack available to provide 360V. Wired in parallel they units drew 25 Amps starting out - fortunately I have a 20Amp circuit in the shop that did not trip during the test. I also had a limited power supply to power the fans, about 5 Amp at 6VDC. I figure that probably corresponds to half speed for the fans. Even at that setting a lot of air was being pumped through the distribution box. The PTC heated up very quickly just like on the bench and after a minute 65C air was blowing out of the distribution box. So that corresponds to a 45C heating of the air which should be enough to heat the cabin or defrost the windshield. All I have left to do is attach the PTC assembly to the distribution box so it does not rattle around when driving the car. I will have to replace all the foam gasket material before the final assembly. I also need to wire up a contactor and a high voltage fuse to connect the battery pack to the heater. A video of this PTC testing can be found here.